When a molded article of a polyolefin resin having relatively low surface polarity, typically polypropylene, is processed by, for example, coating, the surface of the molded article is generally cleaned with a solvent, e.g., trichloroethane prior to priming coat or plasma treatment. Cleaning with a solvent polarizes the surface of the molded article to bring about firm adhesion between the surface and the topcoating paint. However, with the recent increasing need of limiting use of an organic solvent, especially a halogenated hydrocarbon such as trichloroethane, as a cleaning agent, it has been demanded to develop a new approach for surface modification of a polyolefin resin molded article in place of solvent cleaning.
The technique disclosed in JP-A-3-103448 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") is among such approaches. According to this technique, a polypropylene resin molded article is treated with an ozone stream to oxidize the surface and thereby to impart hydrophilic properties.
The problem associated with this method is that uniform modification of all the surface of a resin molded article cannot be achieved without difficulty. That is, in order to modify the surface uniformly, an ozone stream must be applied to all the surface uniformly while equalizing the time of application on every part of the surface. Therefore, where a resin molded article to be treated has a complicated shape, either the molded article or an ozone stream should be moved properly. Otherwise, all the surface of the molded article cannot be modified uniformly, ultimately resulting in a failure of uniform coating.
Besides, the above method still requires cleaning of the surface of a molded article prior to the surface modification to remove any contaminant such as organic substances. The method therefore involves use of an organic solvent for cleaning and, at least, requires an additional step for cleaning.